![MiraCC](https://upload.wikimedia.org/wikipedia/commons/2/2f/MiraCC.jpg)
A variable star is a
star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
whose brightness as seen from
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
(its
apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:
* Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
* Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star
has an orbiting companion that sometimes eclipses it.
Many, possibly most, stars have at least some variation in luminosity: the energy output of the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, for example, varies by about 0.1% over an 11-year
solar cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...
.
Discovery
An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star,
the eclipsing binary
Algol
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
.
Of the modern astronomers, the first variable star was identified in 1638 when
Johannes Holwarda noticed that
Omicron Ceti (later named Mira) pulsated in a cycle taking 11 months; the star had previously been described as a nova by
David Fabricius David Fabricius (9 March 1564 – 7 May 1617) was a German pastor who made two major discoveries in the early days of telescopic astronomy, jointly with his eldest son, Johannes Fabricius (1587–1615).
David Fabricius (Latinization of his proper n ...
in 1596. This discovery, combined with
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
e observed in 1572 and 1604, proved that the starry sky was not eternally invariable as
Aristotle
Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...
and other ancient philosophers had taught. In this way, the discovery of variable stars contributed to the astronomical revolution of the sixteenth and early seventeenth centuries.
The second variable star to be described was the eclipsing variable Algol, by
Geminiano Montanari
Geminiano Montanari (1 June 1633 – 13 October 1687) was an Italian astronomer, lens-maker, and proponent of the experimental approach to science. He was a member of various learned academies, notably the Accademia dei Gelati. Montanari's famous ...
in 1669;
John Goodricke
John Goodricke FRS (17 September 1764 – 20 April 1786) was an English amateur astronomer. He is best known for his observations of the variable star Algol (Beta Persei) in 1782.
Life and work
John Goodricke, named after his great-grandfath ...
gave the correct explanation of its variability in 1784.
Chi Cygni
Chi Cygni (Latinised from χ Cygni) is a Mira variable star in the constellation Cygnus, and also an S-type star. It is around 500 light years away.
χ Cygni is an asymptotic giant branch star, a very cool and luminous red giant nearin ...
was identified in 1686 by
G. Kirch, then
R Hydrae
R Hydrae, abbreviated R Hya, is a single star in the equatorial constellation of Hydra, about 2.7° to the east of Gamma Hydrae. It is a Mira-type variable that ranges in apparent visual magnitude from 3.5 down to 10.9 over a period of ...
in 1704 by
G. D. Maraldi. By 1786, ten variable stars were known. John Goodricke himself discovered
Delta Cephei
Delta Cephei (δ Cep, δ Cephei) is the Bayer designation for a quadruple star system located approximately 887 light-years away in the northern constellation of Cepheus, the King. At this distance, the visual magnitude of the star is ...
and
Beta Lyrae
Beta Lyrae (β Lyrae, abbreviated Beta Lyr, β Lyr) officially named Sheliak (Arabic: الشلياق, Romanization: ash-Shiliyāq) ( IPA: ), the traditional name of the system, is a multiple star system in the constellation of Lyra. Ba ...
. Since 1850, the number of known variable stars has increased rapidly, especially after 1890 when it became possible to identify variable stars by means of photography.
The latest edition of the
General Catalogue of Variable Stars
The General Catalogue of Variable Stars (GCVS) is a list of variable stars. Its first edition, containing 10,820 stars, was published in 1948 by the Academy of Sciences of the USSR and edited by B. V. Kukarkin and P. P. Parenago. Second and thi ...
(2008) lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, and over 10,000 'suspected' variables.
Detecting variability
The most common kinds of variability involve changes in brightness, but other types of variability also occur, in particular changes in the
spectrum
A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...
. By combining
light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...
data with observed spectral changes, astronomers are often able to explain why a particular star is variable.
Variable star observations
Variable stars are generally analysed using
photometry Photometry can refer to:
* Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision
* Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electro ...
,
spectrophotometry
Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spec ...
and
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
. Measurements of their changes in brightness can be plotted to produce
light curve
In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...
s. For regular variables, the
period
Period may refer to:
Common uses
* Era, a length or span of time
* Full stop (or period), a punctuation mark
Arts, entertainment, and media
* Period (music), a concept in musical composition
* Periodic sentence (or rhetorical period), a concept ...
of variation and its
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplit ...
can be very well established; for many variable stars, though, these quantities may vary slowly over time, or even from one period to the next. Peak brightnesses in the light curve are known as maxima, while troughs are known as minima.
Amateur astronomers
Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers m ...
can do useful scientific study of variable stars by visually comparing the star with other stars within the same
telescopic field of view of which the magnitudes are known and constant. By estimating the variable's magnitude and noting the time of observation a visual lightcurve can be constructed. The
American Association of Variable Star Observers
The American Association of Variable Star Observers (AAVSO) is an international nonprofit organization, founded in 1911, focused on coordinating, analyzing, publishing, and archiving variable star observations made largely by amateur astronomers. ...
collects such observations from participants around the world and shares the data with the scientific community.
From the light curve the following data are derived:
* are the brightness variations periodical, semiperiodical, irregular, or unique?
* what is the period of the brightness fluctuations?
* what is the shape of the light curve (symmetrical or not, angular or smoothly varying, does each cycle have only one or more than one minima, etcetera)?
From the spectrum the following data are derived:
* what kind of star is it: what is its temperature, its
luminosity class
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
(
dwarf star
A dwarf star is a star of relatively small size and low luminosity. Most main sequence stars are dwarf stars. The meaning of the word "dwarf" was later extended to some star-sized objects that are not stars, and compact stellar remnants which are ...
,
giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence, main-sequence (or ''dwarf'') star of the same effective temperature, surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moo ...
,
supergiant
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...
, etc.)?
* is it a single star, or a binary? (the combined spectrum of a binary star may show elements from the spectra of each of the member stars)
* does the spectrum change with time? (for example, the star may turn hotter and cooler periodically)
* changes in brightness may depend strongly on the part of the spectrum that is observed (for example, large variations in visible light but hardly any changes in the infrared)
* if the wavelengths of spectral lines are shifted this points to movements (for example, a periodical swelling and shrinking of the star, or its rotation, or an expanding gas shell) (
Doppler effect
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
)
* strong magnetic fields on the star betray themselves in the spectrum
* abnormal emission or absorption lines may be indication of a hot stellar atmosphere, or gas clouds surrounding the star.
In very few cases it is possible to make pictures of a stellar disk. These may show darker spots on its surface.
Interpretation of observations
Combining light curves with spectral data often gives a clue as to the changes that occur in a variable star. For example, evidence for a pulsating star is found in its shifting spectrum because its surface periodically moves toward and away from us, with the same frequency as its changing brightness.
About two-thirds of all variable stars appear to be pulsating. In the 1930s astronomer
Arthur Stanley Eddington
Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lumin ...
showed that the mathematical equations that describe the interior of a star may lead to instabilities that cause a star to pulsate. The most common type of instability is related to oscillations in the degree of ionization in outer, convective layers of the star.
When the star is in the swelling phase, its outer layers expand, causing them to cool. Because of the decreasing temperature the degree of ionization also decreases. This makes the gas more transparent, and thus makes it easier for the star to radiate its energy. This in turn makes the star start to contract. As the gas is thereby compressed, it is heated and the degree of ionization again increases. This makes the gas more opaque, and radiation temporarily becomes captured in the gas. This heats the gas further, leading it to expand once again. Thus a cycle of expansion and compression (swelling and shrinking) is maintained.
The pulsation of
cepheids
A Cepheid variable () is a type of star that Instability strip, pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable frequency, period and amplitude.
A strong direct period-l ...
is known to be driven by oscillations in the ionization of
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
(from He
++ to He
+ and back to He
++).
Nomenclature
In a given constellation, the first variable stars discovered were designated with letters R through Z, e.g.
R Andromedae
R Andromedae (R And) is a Mira-type variable star in the constellation Andromeda. Its spectral class is type S because it shows absorption bands of zirconium monoxide (ZrO) in its spectrum. It was among the stars found by Paul Merrill ...
. This system of
nomenclature
Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming conventions, conventions of everyday speech to the i ...
was developed by
Friedrich W. Argelander, who gave the first previously unnamed variable in a constellation the letter R, the first letter not used by
Bayer
Bayer AG (, commonly pronounced ; ) is a German multinational corporation, multinational pharmaceutical and biotechnology company and one of the largest pharmaceutical companies in the world. Headquartered in Leverkusen, Bayer's areas of busi ...
. Letters RR through RZ, SS through SZ, up to ZZ are used for the next discoveries, e.g.
RR Lyrae
RR Lyrae is a variable star in the Lyra constellation, figuring in its west near to Cygnus. As the brightest star in its class, it became the eponym for the RR Lyrae variable class of stars and it has been extensively studied by astro ...
. Later discoveries used letters AA through AZ, BB through BZ, and up to QQ through QZ (with J omitted). Once those 334 combinations are exhausted, variables are numbered in order of discovery, starting with the prefixed V335 onwards.
Classification
Variable stars may be either ''intrinsic'' or ''extrinsic''.
* Intrinsic variable stars: stars where the variability is being caused by changes in the physical properties of the stars themselves. This category can be divided into three subgroups.
** Pulsating variables, stars whose radius alternately expands and contracts as part of their natural evolutionary ageing processes.
** Eruptive variables, stars who experience eruptions on their surfaces like flares or mass ejections.
** Cataclysmic or explosive variables, stars that undergo a cataclysmic change in their properties like
nova
A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...
e and
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
e.
* Extrinsic variable stars: stars where the variability is caused by external properties like rotation or eclipses. There are two main subgroups.
** Eclipsing binaries,
double star
In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.
This occurs because the pair either forms a binary star (i.e. a bi ...
s where, as seen from
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's vantage point the stars occasionally eclipse one another as they orbit.
** Rotating variables, stars whose variability is caused by phenomena related to their rotation. Examples are stars with extreme "sunspots" which affect the apparent brightness or stars that have fast rotation speeds causing them to become ellipsoidal in shape.
These subgroups themselves are further divided into specific types of variable stars that are usually named after their prototype. For example, dwarf novae are designated ''U Geminorum'' stars after the first recognized star in the class, ''U Geminorum''.
Intrinsic variable stars
Examples of types within these divisions are given below.
Pulsating variable stars
Pulsating stars swell and shrink, affecting their brightness and spectrum. Pulsations are generally split into:
radial
Radial is a geometric term of location which may refer to:
Mathematics and Direction
* Vector (geometric), a line
* Radius, adjective form of
* Radial distance, a directional coordinate in a polar coordinate system
* Radial set
* A bearing f ...
, where the entire star expands and shrinks as a whole; and
non-radial, where one part of the star expands while another part shrinks.
Depending on the type of pulsation and its location within the star, there is a natural or
fundamental frequency
The fundamental frequency, often referred to simply as the ''fundamental'', is defined as the lowest frequency of a periodic waveform. In music, the fundamental is the musical pitch of a note that is perceived as the lowest partial present. In ...
which determines the period of the star. Stars may also pulsate in a
harmonic
A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
or
overtone
An overtone is any resonant frequency above the fundamental frequency of a sound. (An overtone may or may not be a harmonic) In other words, overtones are all pitches higher than the lowest pitch within an individual sound; the fundamental i ...
which is a higher frequency, corresponding to a shorter period. Pulsating variable stars sometimes have a single well-defined period, but often they pulsate simultaneously with multiple frequencies and complex analysis is required to determine the separate
interfering
Interference is the act of interfering, invading, or poaching. Interference may also refer to:
Communications
* Interference (communication), anything which alters, modifies, or disrupts a message
* Adjacent-channel interference, caused by extr ...
periods. In some cases, the pulsations do not have a defined frequency, causing a random variation, referred to as
stochastic
Stochastic (, ) refers to the property of being well described by a random probability distribution. Although stochasticity and randomness are distinct in that the former refers to a modeling approach and the latter refers to phenomena themselv ...
. The study of stellar interiors using their pulsations is known as
asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
.
The expansion phase of a pulsation is caused by the blocking of the internal energy flow by material with a high opacity, but this must occur at a particular depth of the star to create visible pulsations. If the expansion occurs below a convective zone then no variation will be visible at the surface. If the expansion occurs too close to the surface the restoring force will be too weak to create a pulsation. The restoring force to create the contraction phase of a pulsation can be pressure if the pulsation occurs in a non-degenerate layer deep inside a star, and this is called an
acoustic or
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
mode of pulsation, abbreviated to
p-mode. In other cases, the restoring force is
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
and this is called a
g-mode
is a Japanese company that specializes in games for Java-compatible mobile phones. The company also licenses content for mobile telecommunications operators, as well as being involved in the original equipment manufacturing of mobile phone games. ...
. Pulsating variable stars typically pulsate in only one of these modes.
Cepheids and cepheid-like variables
This group consists of several kinds of pulsating stars, all found on the
instability strip
The unqualified term instability strip usually refers to a region of the Hertzsprung–Russell diagram largely occupied by several related classes of pulsating variable stars: Delta Scuti variables, SX Phoenicis variables, and rapidly oscillati ...
, that swell and shrink very regularly caused by the star's own mass
resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
, generally by the
fundamental frequency
The fundamental frequency, often referred to simply as the ''fundamental'', is defined as the lowest frequency of a periodic waveform. In music, the fundamental is the musical pitch of a note that is perceived as the lowest partial present. In ...
. Generally the
Eddington valve mechanism for pulsating variables is believed to account for cepheid-like pulsations. Each of the subgroups on the instability strip has a fixed
relationship between period and absolute magnitude, as well as a relation between period and mean density of the star. The period-luminosity relationship was first established for Delta Cepheids by
Henrietta Leavitt
Henrietta Swan Leavitt (; July 4, 1868 – December 12, 1921) was an American astronomer. A graduate of Radcliffe College, she worked at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measu ...
, and makes these high luminosity Cepheids very useful for determining distances to galaxies within the
Local Group
The Local Group is the galaxy group that includes the Milky Way.
It has a total diameter of roughly , and a total mass of the order of .
It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...
and beyond.
Edwin Hubble
Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an Americans, American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.
Hubble proved that many objects ...
used this method to prove that the so-called spiral nebulae are in fact distant galaxies.
Note that the Cepheids are named only for
Delta Cephei
Delta Cephei (δ Cep, δ Cephei) is the Bayer designation for a quadruple star system located approximately 887 light-years away in the northern constellation of Cepheus, the King. At this distance, the visual magnitude of the star is ...
, while a completely separate class of variables is named after
Beta Cephei
Beta Cephei (β Cephei, abbreviated Beta Cep, β Cep) is a triple star system of the third magnitude in the constellation of Cepheus. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 690 light-y ...
.
=Classical Cepheid variables
=
Classical Cepheids (or Delta Cephei variables) are population I (young, massive, and luminous) yellow supergiants which undergo pulsations with very regular periods on the order of days to months. On September 10, 1784,
Edward Pigott
Edward Pigott (1753–1825) was an English astronomer notable for being one of the founders of the study of variable stars.
Biography
Son of the astronomer Nathaniel Pigott, Pigott's work focused on variable stars. Educated in France with ...
detected the variability of
Eta Aquilae
Eta Aquilae (η Aql, η Aquilae) is the Bayer designation for a multiple star in the equatorial constellation of Aquila, the eagle. It was once part of the former constellation Antinous. On average, this star has an apparent visual magnitu ...
, the first known representative of the class of Cepheid variables. However, the namesake for classical Cepheids is the star
Delta Cephei
Delta Cephei (δ Cep, δ Cephei) is the Bayer designation for a quadruple star system located approximately 887 light-years away in the northern constellation of Cepheus, the King. At this distance, the visual magnitude of the star is ...
, discovered to be variable by
John Goodricke
John Goodricke FRS (17 September 1764 – 20 April 1786) was an English amateur astronomer. He is best known for his observations of the variable star Algol (Beta Persei) in 1782.
Life and work
John Goodricke, named after his great-grandfath ...
a few months later.
=Type II Cepheids
=
Type II Cepheids (historically termed W Virginis stars) have extremely regular light pulsations and a luminosity relation much like the δ Cephei variables, so initially they were confused with the latter category. Type II Cepheids stars belong to older
Population II
During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations.
In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926:
Baade noticed ...
stars, than do the type I Cepheids. The Type II have somewhat lower
metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
, much lower mass, somewhat lower luminosity, and a slightly offset period verses luminosity relationship, so it is always important to know which type of star is being observed.
=RR Lyrae variables
=
These stars are somewhat similar to Cepheids, but are not as luminous and have shorter periods. They are older than type I Cepheids, belonging to
Population II
During 1944, Walter Baade categorized groups of stars within the Milky Way into stellar populations.
In the abstract of the article by Baade, he recognizes that Jan Oort originally conceived this type of classification in 1926:
Baade noticed ...
, but of lower mass than type II Cepheids. Due to their common occurrence in
globular cluster
A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...
s, they are occasionally referred to as ''cluster Cepheids''. They also have a well established period-luminosity relationship, and so are also useful as distance indicators. These A-type stars vary by about 0.2–2 magnitudes (20% to over 500% change in luminosity) over a period of several hours to a day or more.
=Delta Scuti variables
=
Delta Scuti (δ Sct) variables are similar to Cepheids but much fainter and with much shorter periods. They were once known as ''Dwarf Cepheids''. They often show many superimposed periods, which combine to form an extremely complex light curve. The typical δ Scuti star has an amplitude of 0.003–0.9 magnitudes (0.3% to about 130% change in luminosity) and a period of 0.01–0.2 days. Their
spectral type
In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...
is usually between A0 and F5.
=SX Phoenicis variables
=
These stars of spectral type A2 to F5, similar to δ Scuti variables, are found mainly in globular clusters. They exhibit fluctuations in their brightness in the order of 0.7 magnitude (about 100% change in luminosity) or so every 1 to 2 hours.
=Rapidly oscillating Ap variables
=
These stars of spectral type A or occasionally F0, a sub-class of δ Scuti variables found on the main sequence. They have extremely rapid variations with periods of a few minutes and amplitudes of a few thousandths of a magnitude.
Long period variables
The long period variables are cool evolved stars that pulsate with periods in the range of weeks to several years.
=Mira variables
=
![Chi Cygni light curve](https://upload.wikimedia.org/wikipedia/commons/8/83/Chi_Cygni_light_curve.png)
Mira variables are
Asymptotic giant branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...
(AGB) red giants. Over periods of many months they fade and brighten by between 2.5 and 11
magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
s, a 6 fold to 30,000 fold change in luminosity.
Mira
Mira (), designation Omicron Ceti (ο Ceti, abbreviated Omicron Cet, ο Cet), is a red-giant star estimated to be 200–400 light-years from the Sun in the constellation Cetus.
ο Ceti is a binary stellar system, consisting of a varia ...
itself, also known as Omicron Ceti (ο Cet), varies in brightness from almost 2nd magnitude to as faint as 10th magnitude with a period of roughly 332 days. The very large visual amplitudes are mainly due to the shifting of energy output between visual and infra-red as the temperature of the star changes. In a few cases, Mira variables show dramatic period changes over a period of decades, thought to be related to the thermal pulsing cycle of the most advanced AGB stars.
=Semiregular variables
=
These are
red giants
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around o ...
or
supergiants
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars ...
. Semiregular variables may show a definite period on occasion, but more often show less well-defined variations that can sometimes be resolved into multiple periods. A well-known example of a semiregular variable is
Betelgeuse
Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion ...
, which varies from about magnitudes +0.2 to +1.2 (a factor 2.5 change in luminosity). At least some of the semi-regular variables are very closely related to Mira variables, possibly the only difference being pulsating in a different harmonic.
=Slow irregular variables
=
These are
red giants
A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around o ...
or
supergiants
Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars ...
with little or no detectable periodicity. Some are poorly studied semiregular variables, often with multiple periods, but others may simply be chaotic.
=Long secondary period variables
=
Many variable red giants and supergiants show variations over several hundred to several thousand days. The brightness may change by several magnitudes although it is often much smaller, with the more rapid primary variations are superimposed. The reasons for this type of variation are not clearly understood, being variously ascribed to pulsations, binarity, and stellar rotation.
Beta Cephei variables
Beta Cephei (β Cep) variables (sometimes called
Beta Canis Majoris
Beta Canis Majoris (β Canis Majoris, abbreviated Beta CMa, β CMa), also named Mirzam , is a star in the southern constellation of Canis Major, the "Great Dog", located at a distance of about 500 light-years (150 parsecs) from ...
variables, especially in Europe) undergo short period pulsations in the order of 0.1–0.6 days with an amplitude of 0.01–0.3 magnitudes (1% to 30% change in luminosity). They are at their brightest during minimum contraction. Many stars of this kind exhibits multiple pulsation periods.
Slowly pulsating B-type stars
Slowly pulsating B (SPB) stars are hot main-sequence stars slightly less luminous than the Beta Cephei stars, with longer periods and larger amplitudes.
Very rapidly pulsating hot (subdwarf B) stars
The prototype of this rare class is
V361 Hydrae
This is the list of notable stars in the constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inan ...
, a 15th magnitude
subdwarf B star. They pulsate with periods of a few minutes and may simultaneous pulsate with multiple periods. They have amplitudes of a few hundredths of a magnitude and are given the GCVS acronym RPHS. They are
p-mode pulsators.
PV Telescopii variables
Stars in this class are type Bp supergiants with a period of 0.1–1 day and an amplitude of 0.1 magnitude on average. Their spectra are peculiar by having weak
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
while on the other hand
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
lines are extra strong, a type of
Extreme helium star.
RV Tauri variables
These are yellow supergiant stars (actually low mass post-AGB stars at the most luminous stage of their lives) which have alternating deep and shallow minima. This double-peaked variation typically has periods of 30–100 days and amplitudes of 3–4 magnitudes. Superimposed on this variation, there may be long-term variations over periods of several years. Their spectra are of type F or G at maximum light and type K or M at minimum brightness. They lie near the instability strip, cooler than type I Cepheids more luminous than type II Cepheids. Their pulsations are caused by the same basic mechanisms related to helium opacity, but they are at a very different stage of their lives.
Alpha Cygni variables
Alpha Cygni (α Cyg) variables are nonradially pulsating supergiants of
spectral class
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
es B
ep to A
epIa. Their periods range from several days to several weeks, and their amplitudes of variation are typically of the order of 0.1 magnitudes. The light changes, which often seem irregular, are caused by the superposition of many oscillations with close periods.
Deneb
Deneb () is a first-magnitude star in the constellation of Cygnus, the swan. Deneb is one of the vertices of the asterism known as the Summer Triangle and the "head" of the Northern Cross. It is the brightest star in Cygnus and the ...
, in the constellation of
Cygnus is the prototype of this class.
Gamma Doradus variables
Gamma Doradus (γ Dor) variables are non-radially pulsating main-sequence stars of
spectral classes
In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...
F to late A. Their periods are around one day and their amplitudes typically of the order of 0.1 magnitudes.
Pulsating white dwarfs
These non-radially pulsating stars have short periods of hundreds to thousands of seconds with tiny fluctuations of 0.001 to 0.2 magnitudes. Known types of pulsating white dwarf (or pre-white dwarf) include the ''DAV'', or ''
ZZ Ceti'', stars, with hydrogen-dominated atmospheres and the spectral type DA;
''DBV'', or ''
V777 Her'', stars, with helium-dominated atmospheres and the spectral type DB;
and ''
GW Vir
PG 1159-035 is the prototypical PG 1159 star after which the class of PG 1159 stars was named. It was discovered in the Palomar-Green survey of ultraviolet-excess stellar astronomical object, objects and, like the other PG 1159 stars, is ...
'' stars, with atmospheres dominated by helium, carbon, and oxygen. GW Vir stars may be subdivided into ''DOV'' and ''PNNV'' stars.
Solar-like oscillations
The
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
oscillates with very low amplitude in a large number of modes having periods around 5 minutes. The study of these oscillations is known as
helioseismology
Helioseismology, a term coined by Douglas Gough, is the study of the structure and dynamics of the Sun through its oscillations. These are principally caused by sound waves that are continuously driven and damped by convection near the Sun's surfa ...
. Oscillations in the Sun are driven stochastically by
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
in its outer layers. The term
solar-like oscillations Solar-like oscillations are oscillations in stars that are excited in the same way as those in the Sun, namely by turbulent convection in its outer layers. Stars that show solar-like oscillations are called solar-like oscillators. The oscillations a ...
is used to describe oscillations in other stars that are excited in the same way and the study of these oscillations is one of the main areas of active research in the field of
asteroseismology
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperature ...
.
BLAP variables
A Blue Large-Amplitude Pulsator (BLAP) is a pulsating star characterized by changes of 0.2 to 0.4 magnitudes with typical periods of 20 to 40 minutes.
Fast yellow pulsating supergiants
A fast yellow pulsating supergiant (FYPS) is a luminous yellow supergiant with pulsations shorter than a day. They are thought to have evolved beyond a red supergiant phase, but the mechanism for the pulsations is unknown. The class was named in 2020 through analysis of
TESS Tess or TESS may refer to:
Music
* Tess (band), a Spanish pop band active from 2000 to 2005
* TESS (musician), a UK musician
Film and theatre
* ''Tess'' (1979 film), a 1979 film adaptation of '' Tess of the d'Urbervilles''
* ''Tess'' (2016 film ...
observations.
Eruptive variable stars
Eruptive variable stars show irregular or semi-regular brightness variations caused by material being lost from the star, or in some cases being accreted to it. Despite the name these are not explosive events, those are the cataclysmic variables.
Protostars
Protostars are young objects that have not yet completed the process of contraction from a gas nebula to a veritable star. Most protostars exhibit irregular brightness variations.
=Herbig Ae/Be stars
=
![V1025 Tauri Taurus Molecular Nebula from the Mount Lemmon SkyCenter Schulman Telescope courtesy Adam Block](https://upload.wikimedia.org/wikipedia/commons/c/c7/V1025_Tauri_Taurus_Molecular_Nebula_from_the_Mount_Lemmon_SkyCenter_Schulman_Telescope_courtesy_Adam_Block.jpg)
Variability of more massive (2–8
solar mass)
Herbig Ae/Be stars is thought to be due to gas-dust clumps, orbiting in the circumstellar disks.
=Orion variables
=
Orion variables are young, hot
pre–main-sequence star
A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of ...
s usually embedded in nebulosity. They have irregular periods with amplitudes of several magnitudes. A well-known subtype of Orion variables are the
T Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri, but it i ...
variables. Variability of
T Tauri star
T Tauri stars (TTS) are a class of variable stars that are less than about ten million years old. This class is named after the prototype, T Tauri, a young star in the Taurus star-forming region. They are found near molecular clouds and iden ...
s is due to spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks.
=FU Orionis variables
=
These stars reside in reflection nebulae and show gradual increases in their luminosity in the order of 6 magnitudes followed by a lengthy phase of constant brightness. They then dim by 2 magnitudes (six times dimmer) or so over a period of many years. ''
V1057 Cygni
V1057 Cygni is a suspected binary star system in the northern constellation of Cygnus. It is a variable star of the FU Orionis-type, and was the second FU Orionis-type variable to be discovered. The system is located at a distance o ...
'' for example dimmed by 2.5 magnitude (ten times dimmer) during an eleven-year period. FU Orionis variables are of spectral type A through G and are possibly an evolutionary phase in the life of ''
T Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri, but it i ...
'' stars.
Giants and supergiants
Large stars lose their matter relatively easily. For this reason variability due to eruptions and mass loss is fairly common among giants and supergiants.
=Luminous blue variables
=
Also known as the
S Doradus variables, the most luminous stars known belong to this class. Examples include the
hypergiant
A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term ''hypergiant'' is defined as luminosity class 0 (zero) in the MK ...
s
η Carinae
Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around distant in t ...
and
P Cygni
P Cygni (34 Cygni) is a variable star in the constellation Cygnus. The designation "P" was originally assigned by Johann Bayer in ''Uranometria'' as a nova. Located about 5,300 light-years (1,560 parsecs) from Earth, it is a hypergian ...
. They have permanent high mass loss, but at intervals of years internal pulsations cause the star to exceed its Eddington limit and the mass loss increases hugely. Visual brightness increases although the overall luminosity is largely unchanged. Giant eruptions observed in a few LBVs do increase the luminosity, so much so that they have been tagged
supernova impostor
Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and gia ...
s, and may be a different type of event.
=Yellow hypergiants
=
These massive evolved stars are unstable due to their high luminosity and position above the instability strip, and they exhibit slow but sometimes large photometric and spectroscopic changes due to high mass loss and occasional larger eruptions, combined with secular variation on an observable timescale. The best known example is
Rho Cassiopeiae
Rho Cassiopeiae (; ρ Cas, ρ Cassiopeiae) is a yellow hypergiant star in the constellation Cassiopeia. It is about from Earth, yet can still be seen by the naked eye as it is over 300,000 times brighter than the Sun. On average it has a ...
.
=R Coronae Borealis variables
=
While classed as eruptive variables, these stars do not undergo periodic increases in brightness. Instead they spend most of their time at maximum brightness, but at irregular intervals they suddenly fade by 1–9 magnitudes (2.5 to 4000 times dimmer) before recovering to their initial brightness over months to years. Most are classified as yellow supergiants by luminosity, although they are actually post-AGB stars, but there are both red and blue giant R CrB stars.
R Coronae Borealis
R Coronae Borealis is a low-mass yellow supergiant star in the constellation of Corona Borealis. It is the prototype of the R Cor Bor class of variable stars, which fade by several magnitudes at irregular intervals. R Coronae Boreali ...
(R CrB) is the prototype star.
DY Persei variable
DY Persei variables are a subclass of R Coronae Borealis (R CrB) variables. They are carbon-rich asymptotic giant branch (AGB) stars that exhibit pulsational variability of AGB stars and irregular fades similar to R CrB stars.
The star DY Per ...
s are a subclass of R CrB variables that have a periodic variability in addition to their eruptions.
Wolf–Rayet variables
Classic population I Wolf–Rayet stars are massive hot stars that sometimes show variability, probably due to several different causes including binary interactions and rotating gas clumps around the star. They exhibit broad emission line spectra with
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
lines. Variations in some stars appear to be stochastic while others show multiple periods.
Gamma Cassiopeiae variables
Gamma Cassiopeiae
Gamma Cassiopeiae, Latinized from γ Cassiopeiae, is a bright star at the center of the distinctive "W" asterism in the northern circumpolar constellation of Cassiopeia. Although it is a fairly bright star with an apparent visual mag ...
(γ Cas) variables are non-supergiant fast-rotating B class emission line-type stars that fluctuate irregularly by up to 1.5 magnitudes (4 fold change in luminosity) due to the ejection of matter at their
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
ial regions caused by the rapid rotational velocity.
Flare stars
In main-sequence stars major eruptive variability is exceptional. It is common only among the
flare star
A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in th ...
s, also known as the
UV Ceti
Luyten 726-8, also known as Gliese 65, is a binary star system that is one of Earth's nearest neighbors, at about 8.7 light years from Earth in the constellation Cetus. The two component stars are both flare stars with the variable st ...
variables, very faint main-sequence stars which undergo regular flares. They increase in brightness by up to two magnitudes (six times brighter) in just a few seconds, and then fade back to normal brightness in half an hour or less. Several nearby red dwarfs are flare stars, including
Proxima Centauri
Proxima Centauri is a small, low-mass star located away from the Sun in the southern constellation of Centaurus. Its Latin name means the 'nearest tarof Centaurus'. It was discovered in 1915 by Robert Innes and is the nearest-kno ...
and
Wolf 359
Wolf 359 is a red dwarf star located in the constellation Leo, near the ecliptic. At a distance of approximately 7.9 light years from Earth, it has an apparent magnitude of 13.54 and can only be seen with a large telescope. Wolf 359 i ...
.
RS Canum Venaticorum variables
These are close binary systems with highly active chromospheres, including huge sunspots and flares, believed to be enhanced by the close companion. Variability scales ranges from days, close to the orbital period and sometimes also with eclipses, to years as sunspot activity varies.
Cataclysmic or explosive variable stars
Supernovae
Supernovae are the most dramatic type of cataclysmic variable, being some of the most energetic events in the universe. A supernova can briefly emit as much energy as an entire
galaxy
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
, brightening by more than 20 magnitudes (over one hundred million times brighter). The supernova explosion is caused by a white dwarf or a star core reaching a certain mass/density limit, the
Chandrasekhar limit
The Chandrasekhar limit () is the maximum mass of a stable white dwarf star. The currently accepted value of the Chandrasekhar limit is about ().
White dwarfs resist gravitational collapse primarily through electron degeneracy pressure, compared ...
, causing the object to collapse in a fraction of a second. This collapse "bounces" and causes the star to explode and emit this enormous energy quantity. The outer layers of these stars are blown away at speeds of many thousands of kilometers per second. The expelled matter may form nebulae called ''
supernova remnant
A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
s''. A well-known example of such a nebula is the
Crab Nebula
The Crab Nebula (catalogue designations Messier object, M1, New General Catalogue, NGC 1952, Taurus (constellation), Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus (constellation), Taurus. The common name ...
, left over from a supernova that was observed in
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
and elsewhere in 1054. The progenitor object may either disintegrate completely in the explosion, or, in the case of a massive star, the core can become a
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
(generally a
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
).
Supernovae can result from the death of an extremely massive star, many times heavier than the Sun. At the end of the life of this massive star, a non-fusible iron core is formed from fusion ashes. This iron core is pushed towards the Chandrasekhar limit till it surpasses it and therefore collapses. One of the most studied supernovae of this type is
SN 1987A
SN 1987A was a type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. It occurred approximately from Earth and was the closest observed supernova since Kepler's Supernova. 1987A's light reached Earth on Feb ...
in the
Large Magellanic Cloud
The Large Magellanic Cloud (LMC), or Nubecula Major, is a satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (≈160,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the ...
.
A supernova may also result from mass transfer onto a
white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
from a star companion in a double star system. The Chandrasekhar limit is surpassed from the infalling matter. The absolute luminosity of this latter type is related to properties of its light curve, so that these supernovae can be used to establish the distance to other galaxies.
Luminous red nova
Luminous red novae are stellar explosions caused by the merger of two stars. They are not related to classical
novae
A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...
. They have a characteristic red appearance and very slow decline following the initial outburst.
Novae
Nova
A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...
e are also the result of dramatic explosions, but unlike supernovae do not result in the destruction of the progenitor star. Also unlike supernovae, novae ignite from the sudden onset of thermonuclear fusion, which under certain high pressure conditions (
degenerate matter
Degenerate matter is a highly dense state of fermionic matter in which the Pauli exclusion principle exerts significant pressure in addition to, or in lieu of, thermal pressure. The description applies to matter composed of electrons, protons, neu ...
) accelerates explosively. They form in close
binary system
A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that th ...
s, one component being a white dwarf accreting matter from the other ordinary star component, and may recur over periods of decades to centuries or millennia. Novae are categorised as ''fast'', ''slow'' or ''very slow'', depending on the behaviour of their light curve. Several
naked eye
Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...
novae have been recorded,
Nova Cygni 1975
__NOTOC__
V1500 Cygni or Nova Cygni 1975 was a bright nova occurring in 1975 in the constellation Cygnus. It had the second highest intrinsic brightness of any nova of the 20th century, exceeded only by CP Puppis in 1942.
V1500 Cygni ...
being the brightest in the recent history, reaching 2nd magnitude.
Dwarf novae
Dwarf novae are double stars involving a
white dwarf
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
in which matter transfer between the component gives rise to regular outbursts. There are three types of dwarf nova:
*
U Geminorum star
A U Geminorum-type variable star, or dwarf nova (pl. novae) is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dwa ...
s, which have outbursts lasting roughly 5–20 days followed by quiet periods of typically a few hundred days. During an outburst they brighten typically by 2–6 magnitudes. These stars are also known as
SS Cygni variable
A U Geminorum-type variable star, or dwarf nova (pl. novae) is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dwa ...
s after the variable in
Cygnus which produces among the brightest and most frequent displays of this variable type.
*
Z Camelopardalis star
A U Geminorum-type variable star, or dwarf nova (pl. novae) is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dwa ...
s, in which occasional plateaux of brightness called ''standstills'' are seen, part way between maximum and minimum brightness.
*
SU Ursae Majoris star
A U Geminorum-type variable star, or dwarf nova (pl. novae) is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dwa ...
s, which undergo both frequent small outbursts, and rarer but larger ''
superoutbursts''. These binary systems usually have orbital periods of under 2.5 hours.
DQ Herculis variables
DQ Herculis systems are interacting binaries in which a low-mass star transfers mass to a highly magnetic white dwarf. The white dwarf spin period is significantly shorter than the binary orbital period and can sometimes be detected as a photometric periodicity. An accretion disk usually forms around the white dwarf, but its innermost regions are magnetically truncated by the white dwarf. Once captured by the white dwarf's magnetic field, the material from the inner disk travels along the magnetic field lines until it accretes. In extreme cases, the white dwarf's magnetism prevents the formation of an accretion disk.
AM Herculis variables
In these cataclysmic variables, the white dwarf's magnetic field is so strong that it synchronizes the white dwarf's spin period with the binary orbital period. Instead of forming an accretion disk, the accretion flow is channeled along the white dwarf's magnetic field lines until it impacts the white dwarf near a magnetic pole. Cyclotron radiation beamed from the accretion region can cause orbital variations of several magnitudes.
Z Andromedae variables
These symbiotic binary systems are composed of a red giant and a hot blue star enveloped in a cloud of gas and dust. They undergo nova-like outbursts with amplitudes of up to 4 magnitudes. The prototype for this class is
Z Andromedae
Z Andromedae is a binary star system consisting of a red giant and a white dwarf. It is the prototype of a type of cataclysmic variable star known as symbiotic variable stars or simply Z Andromedae variables. The brightness of those st ...
.
AM CVn variables
AM CVn variables are symbiotic binaries where a white dwarf is accreting helium-rich material from either another white dwarf, a helium star, or an evolved main-sequence star. They undergo complex variations, or at times no variations, with ultrashort periods.
Extrinsic variable stars
There are two main groups of extrinsic variables: rotating stars and eclipsing stars.
Rotating variable stars
Stars with sizeable
sunspot
Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
s may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view. Bright spots also occur at the magnetic poles of magnetic stars. Stars with ellipsoidal shapes may also show changes in brightness as they present varying areas of their surfaces to the observer.
Non-spherical stars
=Ellipsoidal variables
=
These are very close binaries, the components of which are non-spherical due to their tidal interaction. As the stars rotate the area of their surface presented towards the observer changes and this in turn affects their brightness as seen from Earth.
Stellar spots
The surface of the star is not uniformly bright, but has darker and brighter areas (like the sun's
solar spots). The star's
chromosphere
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Su ...
too may vary in brightness. As the star rotates we observe brightness variations of a few tenths of magnitudes.
=FK Comae Berenices variables
=
![FKComLightCurve](https://upload.wikimedia.org/wikipedia/commons/2/21/FKComLightCurve.png)
These stars rotate extremely rapidly (~100 km/s at the
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
); hence they are
ellipsoidal
An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation.
An ellipsoid is a quadric surface; that is, a surface that may be defined as the z ...
in shape. They are (apparently) single giant stars with
spectral type
In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...
s G and K and show strong
chromospheric
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the S ...
emission line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s. Examples are
FK Com,
V1794 Cygni
V1794 Cygni is a single variable star in the northern constellation Cygnus (constellation), Cygnus. It has the identifier HD 199178 from the ''Henry Draper Catalogue''; ''V1794 Cygni'' is its variable star designation. With an apparent vis ...
and
UZ Librae. A possible explanation for the rapid rotation of FK Comae stars is that they are the result of the merger of a
(contact) binary.
= BY Draconis variable stars
=
BY Draconis stars are of spectral class K or M and vary by less than 0.5 magnitudes (70% change in luminosity).
Magnetic fields
= Alpha-2 Canum Venaticorum variables
=
Alpha-2 Canum Venaticorum (α
2 CVn) variables are
main-sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...
stars of spectral class B8–A7 that show fluctuations of 0.01 to 0.1 magnitudes (1% to 10%) due to changes in their magnetic fields.
=SX Arietis variables
=
Stars in this class exhibit brightness fluctuations of some 0.1 magnitude caused by changes in their magnetic fields due to high rotation speeds.
=Optically variable pulsars
=
Few
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
s have been detected in
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
. These
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
s change in brightness as they rotate. Because of the rapid rotation, brightness variations are extremely fast, from milliseconds to a few seconds. The first and the best known example is the
Crab Pulsar
The Crab Pulsar (PSR B0531+21) is a relatively young neutron star. The star is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was widely observed on Earth in the year 1054.[binary star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...]
. When seen from certain angles, one star may
eclipse
An eclipse is an astronomical event that occurs when an astronomical object or spacecraft is temporarily obscured, by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three ce ...
the other, causing a reduction in brightness. One of the most famous eclipsing binaries is
Algol
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
, or Beta Persei (β Per).
Algol variables
Algol variables undergo eclipses with one or two minima separated by periods of nearly constant light. The prototype of this class is
Algol
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
in the
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
Perseus
In Greek mythology, Perseus (Help:IPA/English, /ˈpɜːrsiəs, -sjuːs/; Greek language, Greek: Περσεύς, Romanization of Greek, translit. Perseús) is the legendary founder of Mycenae and of the Perseid dynasty. He was, alongside Cadmus ...
.
Double Periodic variables
Double periodic variables exhibit cyclical mass exchange which causes the orbital period to vary predictably over a very long period. The best known example is
V393 Scorpii.
Beta Lyrae variables
Beta Lyrae (β Lyr) variables are extremely close binaries, named after the star
Sheliak. The light curves of this class of eclipsing variables are constantly changing, making it almost impossible to determine the exact onset and end of each eclipse.
W Serpentis variables
W Serpentis is the prototype of a class of semi-detached binaries including a giant or supergiant transferring material to a massive more compact star. They are characterised, and distinguished from the similar β Lyr systems, by strong UV emission from accretions hotspots on a disc of material.
W Ursae Majoris variables
The stars in this group show periods of less than a day. The stars are so closely situated to each other that their surfaces are almost in contact with each other.
Planetary transits
Stars with
planets
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a young ...
may also show brightness variations if their planets pass between Earth and the star. These variations are much smaller than those seen with stellar companions and are only detectable with extremely accurate observations. Examples include
HD 209458
HD 209458 is an 8th- magnitude star in the constellation Pegasus. It is a G0V star, and is thus very similar to the Sun. Because it is located at a distance of about 159 light years, it is not visible to the unaided eye. With good bino ...
and
GSC 02652-01324
GSC 02652-01324 is an orange dwarf main sequence star approximately 523 light-years away in the constellation of Lyra (the Lyre).
Planetary system
In 2004 the extrasolar planet TrES-1b was found to be orbiting this star by the Trans-At ...
, and all of the planets and planet candidates detected by the
Kepler Mission
The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
.
See also
*
Guest star
In show business, a guest appearance is the participation of an outsider performer (such as a musician or actor) in an event such as a music record or concert, show, etc., when the performer does not belong to the regular band, cast, or other ...
*
Irregular variable
An irregular variable is a type of variable star in which variations in brightness show no regular periodicity. There are two main sub-types of irregular variable: eruptive and pulsating.
Eruptive irregular variables are divided into three categor ...
*
List of variable stars
, there are over 52,011 known variable stars, with more being discovered regularly, so a complete list of every single variable is impossible at this place (cf. GCVS). The following is a list of variable stars that are well-known, bright, signifi ...
*
Low-dimensional chaos in stellar pulsations
Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. These fluctuations in stellar radius cause corresponding changes in the luminosity of the star. Astronomers are able to ded ...
*
Stellar pulsations
Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain equilibrium. These fluctuations in stellar radius cause corresponding changes in the luminosity of the star. Astronomers are able to ded ...
References
External links
The American Association of Variable Star ObserversGCVS Variability TypesSociety for Popular Astronomy – Variable Star Section
{{DEFAULTSORT:Variable Star
Star types
Concepts in astronomy